HP and Epson printers are drop on demand printers. They use electrical pulses to complete requests for ink. The ink is barred out of the print head by a bubble which forms as a reaction to being heated. Once the ink is push out of the cartridge, the bubble cools, contracts, and forms a vacuum of sorts, which pulls more ink into the nozzle.

Epson printers work on the same principal with the exception of a slight twist they use a transducer to produce the droplets in what is referred to as piezoelectric DOD.thus what about the ink does it matter what kind you buy as a substitute?
The ingredients that go into ink play a huge role in the quality of your printouts. All ink producer work to hit the delicate balance between print quality, time it takes to dry, and needle performance. As you might have guessed, as the print quality improves, the dry time increases.

Producer have to be careful that the ink does not dry too quickly there is the danger that it will really dry on the nozzle. This can ruin your print head, absolutely clogging up your nozzle, and reducing the print quality.